1. Field of the Invention
The present invention relates to a wavefront measuring apparatus and method for evaluating the optical performance of an optical system (hereinafter referred to as “test optical system”) having a lens, a prism or other optical element. More particularly, the present invention relates to a wavefront measuring apparatus and method capable of conducting an optical performance evaluation without being affected by unwanted reflected light or irrelevant aberrations even when the test optical system is an immersion optical system.
2. Discussion of Related Art
A conventional measuring apparatus used to evaluate the performance of a test optical system is shown in FIG. 8. The measuring apparatus has a Fizeau interferometric optical system, which comprises a light source 1, a light-collecting lens 2, a pinhole 3, a beam splitter 4, a collimator lens 5, a semitransparent mirror 6 as a reference member, a concave member 8, a spatial filter 9, a relay lens 10, and a CCD camera 11 as an image pickup device. An objective 7 as a test optical system is placed between the semitransparent mirror 6 and the concave member 8.
Light emitted from the light source 1 is once collected on the pinhole 3 through the light-collecting lens 2. Light passing through the aperture of the pinhole 3 is formed into a parallel beam through the collimator lens 5 and incident on the semitransparent mirror 6. The semitransparent mirror 6 produces reflected light and transmitted light according to a predetermined reflectance (or transmittance). Of the light incident on the semitransparent mirror 6, light reflected from the semitransparent mirror 6 is herein referred to as “reference light”. The reference light passes through the collimator lens 5 and is then reflected by the beam splitter 4. Then, the reference light passes successively through the spatial filter 9 and the relay lens 10 to enter the CCD camera 11.
Meanwhile, light passing through the semitransparent mirror 6 enters the objective 7. Herein, light passing through the test optical system (objective 7) is referred to as “test light”. If the objective 7 has aberrations, the wavefront of the test light is deformed. After being collected through the objective 7, the test light diverges as it is incident on the concave member 8. The concave surface of the concave member 8 has such a curvature that the direction of reflection of the incident light is coincident with the direction of incidence of the light. Accordingly, the test light reflected from the concave surface of the concave member 8 reenters the objective 7 and exits therefrom in the form of a parallel beam. The parallel beam exiting from the objective 7 passes through the semitransparent mirror 6 and enters the CCD camera 11 in the same way as the reference light.
Light reflected from the beam splitter 4 includes the reference light and the test light. Therefore, interference occurs between the reference light and the test light, and interference fringes are formed on the CCD camera 11 by the relay lens 10. Thus, the state of the interference can be observed. It should be noted that interference fringes suitable for measurement can be obtained by moving the semitransparent mirror 6 along the optical axis. The interference fringes formed on the CCD camera 11 contain information concerning aberrations of the objective 7. Therefore, it is possible to obtain aberrations of the objective 7, e.g. wavefront aberrations, by analyzing the interference fringes.
It should be noted that, in the arrangement shown in FIG. 8, the optical path through which the reference light passes, i.e. from the semitransparent mirror 6 to the beam splitter 4 (or the CCD camera 11), corresponds to the reference light path. The optical path through which the test light passes, i.e. from the concave member 8 to the beam splitter 4 (or the CCD camera 11), corresponds to the test light path. The optical path from the light source 1 to the beam splitter 4 is a common light path. The optical path from the beam splitter 4 to the CCD camera 11 may also be said to be a common light path.
A similar technique of measuring the optical characteristics of a test optical system by utilizing an interferometric optical system as in the case of FIG. 8 is disclosed in Japanese Patent Application Unexamined Publication Number [hereinafter referred to as “JP(A)”] Hei 10-90113. In JP(A) Hei 10-90113, a hemispherical lens is used in place of the concave member 8. The reason for using the hemispherical lens is that the divergence angle of the beam can be reduced according to the refractive index of the vitreous material of the hemispherical lens. Thus, the optical member (the concave member 8 in FIG. 8 or the hemispherical lens in JP(A) Hei 10-90113) for reflecting the light exiting from the test lens back to it can be produced in a compact and lightweight structure. Further, it is possible to dispense with a compensating plate by taking into consideration the cover glass thickness when setting the thickness of the hemispherical lens.
JP(A) Hei 9-184787 discloses a technique that allows measurement of a test lens even when it is an immersion objective. In JP(A) Hei 9-184787, three test lenses are prepared, and wavefront measurement is carried out for each pair of the three lenses in such a manner that the two lenses are placed to face each other. In this way, a total of three combinations of the lenses are subjected to wavefront measurement, and a wavefront is determined by computation. In this case, if coordinate systems used for the measurement of the three combinations of the lenses are not held in a predetermined positional relationship, accurate computation cannot be executed. Therefore, the disclosed technique contrives that the coordinate systems should be held in a predetermined positional relationship. JP(A) Hei 9-184787 discloses that when the test lenses are immersion objectives, the space between the objectives placed to face each other is filled with a liquid.
JP(A) Hei 10-90113 certainly allows the optical element itself to be produced in a compact structure when a hemispherical lens is used, but it has the problem that the influence of reflected light from the plane portion of the hemispherical lens cannot be avoided. For example, as shown in FIG. 1 of JP(A) Hei 10-90113, air is present between the foremost lens element in the test lens and the. hemispherical lens. Therefore, about 4% of light exiting from the test lens and entering the hemispherical lens is reflected at the plane portion of the hemispherical lens. The reflectance at the convex surface of the hemispherical lens is also about 4%.
Accordingly, light reflected from the plane surface of the hemispherical lens, which is not originally necessary for the measurement, is added to the test light reflected from the convex surface of the hemispherical lens. If the reflectance of a plane plate is set at about 4%, the reference light and the test light become approximately equal in intensity to each other. Therefore, interference fringes formed by interference are substantially the same as those which should originally be obtained, and thus have a fairly high contrast. However, if reflected light occurs at the plane portion of the hemispherical lens, this reflected light also interferes with the reference light, together with the test light. At this time, the intensity of the reflected light is approximately equal to that of the test light. Accordingly, the contrast of interference fringes produced by the reflected light is approximately equal to the contrast of interference fringes produced by the reference light and the test light.
Thus, with the arrangement disclosed in JP(A) Hei 10-90113, not only interference fringes produced by the reference light and the test light but also interference fringes produced by the reference light and the light reflected from the plane portion of the hemispherical lens are formed on the CCD camera. These two interference fringe patterns are obtained in the form of coherent summation (summation of amplitudes). Therefore, the two interference fringe patterns cannot be separated from each other after they have been imaged. In other words, it is impossible to remove the interference fringes produced by the reference light and light reflected from the plane portion of the hemispherical lens from the interference fringes produced by the reference light and the test light, which are originally necessary for the measurement. Consequently, accurate wavefront measurement cannot be carried out.
It is conceivable to provide the plane portion with an antireflection coating or the like for the purpose of reducing reflection at the plane portion. However, when the numerical aperture of the test optical system is high, the angle of light incident on the plane surface is about 70° at the maximum. It is very difficult in general practice to provide an antireflection coating capable of making the reflectance nearly zero with respect to a wide range of incident angles, i.e. from 0° to 70°. Further, because air is present between the test lens and the hemispherical lens, if the hemispherical lens deviates from a perfect hemisphere (exclusive of the amount of compensation made by the cover glass), the deviation appears as aberration, making it impossible to perform satisfactory measurement.
On the other hand, the technique disclosed in JP(A) Hei 9-184787 takes into consideration not only a dry optical system but also an immersion optical system as test optical systems but needs at least three sets of optical systems as test optical systems. Further, it is necessary to adjust the coordinate system for each combination of test optical systems. Accordingly, the optical system requires very severe adjustment. Thus, it is difficult to adjust the optical system.